Grain size data for Cowlitz River Sediment
-- From the Toutle to the Columbia
Image from US Army Corps of Engineers report "Mount St. Helens Project Cowlitz River Levee Systems 2009 Level of Flood Protection Update Summary"
http://www.nwp.usace.army.mil/docs/d_msh/Final_Cowlitz_LOP_02-04-10.pdf
The data show an abrupt decrease in median grain size (D50) from gravel to sand at about 8 river miles below the confluence with the Toutle River. The cause may be real: a gravel wedge slowly migrating down the river from sources upstream, or apparent: periodic removal of a sand covering from the upstream section, exposing gravel bars underneath. The strongly fluctuating position of the transition supports the second idea, but both causes may be at work.
Some additional resources:
Other images from US Army Corps of Engineers
Flood frequency data for the Cowlitz at Castle Rock (xlsx)
Geotechnical data for the Castle Rock levee system
Other data sources
http://www.nwp.usace.army.mil/docs/d_msh/Cowlitz_levees_safe_water_level2010.pdf
http://www.nwp.usace.army.mil/docs/d_msh/Toutle-Cowlitz_sediment_budget.pdf